Low-Cost Cabinet Solution for Prototype Systems

Would cabinets made using a custom extrusion technique be good enough (and worth the cost) for creators of prototype electronic products?

Both the creators of hobby projects and the creators of professional prototypes share a common problem -- what to do about the cabinet. In the not-so-distant past, creating a prototype printed circuit board (PCB) was the most time-consuming and expensive problem, but now this is much less of an issue. By comparison, acquiring an affordable, professional-looking cabinet remains an expensive undertaking.

Recently, I've been chatting to someone who thinks he has a solution -- EETimes community member Charles Spence. In a recent telephone conversation, Charles was saying that he is thinking of launching a Kickstarter project, but first he would like to "hoist his idea up the flagpole and see how well it flies," as it were, and that's where you come into the picture.

I personally really like Charles's idea, so I asked him to write it up in more detail so I could share it with you. Charles responded as follows:

Dear Max, thank you for giving me the opportunity to share my idea with other members of the EETimes Community. One part of building prototype electronics that continues to plague me is the cabinet. It still seems to be the remaining piece for the hobbyist and professional that forces their work to look amateurish to family and friends or to the customer. While I admire the quick and dirty methods Jim Williams and Bob Pease taught us, only the enlightened few seemed to deeply appreciate the ingenuity of a cookie tin for EMI shielding. However, it takes an understanding wife or boss to accommodate the hoarding of these likely Christmas gifts.

While I am also a fan of the "dead bug" method of circuit prototyping, it tends to hide significant issues with regard to parasitic capacitance or leakage paths that -- if you ever want to replicate your circuit -- will bite you. I also argue that creating a decent cabinet is more time-consuming than creating a prototype PCB. This is because you can go straight to a PCB for little relative cost these days (when I started my first real electronics job, it was $1K for a set of two-layer proto-PCBs; now it's only around $120 on the high end). I create a PCB for all my prototypes -- what seems to remain is the box.

On my last project, I think I came up with a solution I would like to share with your readers. This solution applies principles similar to those that enable low-cost prototype PCBs and even low-cost integrated circuits -- share the panel or wafer.

Current industrial CNC laser cutters can slice up a panel of aluminum quickly and cheaply. What seems to plague the sheet metal business is making a bend cheaply enough for use in prototype cabinets. A simple aluminum extrusion gets past this issue, which inspired the following cabinet design.

This example represents a 3" x 5" x 7" deep cabinet. The panels are intended to be cut from 0.050" thick 5052-H32 sheet aluminum. For the purposes of this example, the extrusions are shown in orange in an effort to make their design more visible. These extrusions are intended to be made from 6061 aluminum hardened to T5 or T6. The extrusions do include a guide channel for a PCB, but the PCB(s) could also be mounted to any panel via standoffs and screws.

In one implementation, the bottom and two side panels could be glued to the extrusions with a cyanoacrylate (Krazy Glue I) like Loctite 380 (Black Max) or an adhesive transfer tape like 3M 465 or 966. The top panel would still be allowed to slide to facilitate easier access to the internal electronics for debugging and testing.

The cheapest way to make this type of cabinet uses raw aluminum. My current estimates show they could be made with custom cutouts for $50 each in quantities of one, assuming several people ordered cabinets at the same time (each cabinet can be different). Furthermore, since the un-assembled cabinet can be shipped flat, it can be sent via USPS Priority Mail bubble pack to most places in the US for less than $6.00.

Observe that this basic design can be extended all the way up to 19" rack-mount cabinets. The front panel can be extended to have rack mounting holes (but it would also need to be made from thicker aluminum, which would drive the price up a bit). A simple bracket could be mounted in the middle to stiffen the center section for cabinets this wide.

Never had a dremel type cut-off wheel explode with Al either. I just remember the stern warnings (or worse) of a couple shop teachers. Something about the Al melting into the abrasive of the wheel and causing stress cracks from expansion and contraction. Cut-off wheels may wear too fast to matter. I felt I should be a nervous ninny to remind folks. And thank you for the Safety Glasses.

Wood cabinets are the best aesthetic and flexible. The tools and material are readily available. Remember all those 60's "high end" (ha! ha!) console tv/stereo combos. Max is into these for some of his projects. However, for most of my protos I worry about some boneheaded error on my part and sparking things (fire) and EMI. Also protecting others who eventually use it. I have done a lot of low noise analog and needed the shielding.

McMaster has a good selection of extruded plastic of standard profiles. (I made a PMT housing from PVC pipe. Got a few laughs on that one.)

I appreciate the support. I will keep folks posted. Still appreciate the continuing feedback.

I've not had any of the inexpensive 4" metal cutting wheels from HF explode Charles, and I've cut all sorts of stuff up :-) Turned an old hot water heater into a rain water cistern with them, cut pipe, rebar... Good reason to remember to wear the safety goggles though.

I don't have any concept of the costs of a tool for any sort of extrusion, so trust your comments entirely on the economics of such things. I CAN tell you that you can make the equivalent of very simple corner extrusions out of wood on a table saw or router (although I've only done this for boxes made of 1/8" plywood and fiberboard). It makes a box that looks better than some alternatives I've tried, but of course not useful in your basic home radar project.

Can you give me a pointer to the stock plastic estrusions you mention?

Anyway -- I think your onto something if you can make the numbers work, and I hope you'll keep us posted if you go forward.

At the risk of stating the obvious, you need a certain volume of orders to make any of it work even if you just broke even.

I have no problems with plastic extrusions. In fact the case pictured in the blog piece is intended to use stock vinyl right angle extrusions in an attempt to solve the sharp edges of the aluminum panels on four of the corners. But there are still eight left for the front and back panels. I eventually came up with a solution for that using plastic extrusions and molded corner pieces but ..., dies, molds, etc.

I did not pursue the cost of plastic extrusion dies. I imagine they are in the ballpark of aluminum extrusion dies if they have to be cut with EDM. Injection mold dies start at $3K, maybe half overseas, then there is setup and run costs. You can cast plastic parts for short runs but you run into a labor issue that seems to make it a wash.

You could also coat the plastic extrusions to assist with EMI and probably look better than copper tape. Again, yet another setup cost to make this viable.

Laser cut arcrylic with a can of conductive shielding paint may be the cheapest EMI shielded custom construction you can get now that looks descent. You can see examples here:

Looks straight forward to incorporate the plots mentioned for the front panels.

Oh I love my HF grinder and cut-off wheels. Just don't like the sweat under the ear protectors (ear plugs need clean hands). Little scared with Al. Do not know if the cheap HF cut-off wheels have explosive tendencies like non-Al rated grinding wheels do?

The inspiration for this cabinet construction was finding an industrial laser CNC job shop that charged straight setup and cut time. The vendor before that was charging me three times this cost. I then did the numbers to see what a service based on this fabrication method might cost. Could not find the stock extrusions till recently so I designed one and costed that out. Looked promising.

The end design intends to be a reasonable compromise attempting to satisfy a broad base (colored by my skewed perspective) of applications. Mechanically strong, adaptable to a wide range of sizes, some amount of EMI protection and safety grounding, not dramatically more than low end stock cases, customizable and looks professional.

While a conductive coated plastic case solves many EMI issues (they use it all over laptops) it presents a safety concern. Not a great path to an earth ground if you bring AC power into the case. Plastic, especially what you can get easily and cheaply, is relatively weak. Yeah, I know polycarbonite, but unless you glue it you will use mechanical fasteners and it cracks on me all the time.

I greatly appreciate the input. I love being reminded of the practicality, cost effectiveness and ingenuity of food containers for electronic prototypes. Makes me crazy. I wish I could come up with an even cheaper solution to bridge this apparent $50 or better gap to making it "look good."

I like the idea of being able to purchase the extrusions in longer lengths and cutting them to size myself, along perhaps with a selection of standard-sized panels (metal and plastic panels would both be useful).

I'd also add that a lot of my projects could get by nicely with nylon extrusions rather than aluminium. Copper tape is inexpensive and can be used to enhance shielding where needed. It would be a big step up from my usual resorts (my last project went into a sardine tin :-)

I use my inexpensive CAD software to draw my front panels, and then plot or print them in color. IThe plots get mounted under an acrylic panel (storm window stuff) to keep them clean. It works for me, and you can't beat the price.

An aside: you can buy a 4" electrical grinder and a set of metal cut-off wheels for it from Harbor Freight Tools for under $20 I think - and it beats the heck out of a hacksaw when the workshop temperature is in the high 90s. Build your own table for it. The cut-off wheels work great on fiberglass copperclad too (avoid breathing the dust).

In my efforts to find other options I discovered one vendor that currently offers a service similar to what I proposed, Front Panel Express in Seattle, WA. However, their price point appears only slightly less than current prototype sheet metal vendors. I would argue that their cabinets look better and include front panel labeling. They also sell extrusions separately that are similar to what I propose and what antedeluvian had referenced from Bopla amoung others.

They also offer a software tool that appears focused on front panel layout and incorporates their pricing and ordering functions. It appears to offer much of what Max suggested. It is not clear how well it implements the spectrum of connectors, switches, etc. to test for interference and fits. I have not tried it. I think that is a lot to ask of a free tool sponsored by one vendor of this size. It appears you must order the extrusions through this tool to get pricing.

I have a jigsaw and a drill as well. It always has that "professional" look when I am done (holding up sarcasm sign). The intent of what I propose is that for essentially the same amount of work and likely overall cost you could have a pretty cabinet if a services such as I proposed existed. I have butchered my share of LMB boxes over the years using the jigsaw and drill. Unless I put a full size sticker on the front it looks like hell. Even then it looks hokey to me

When Advanced Circuits and Eagle finally came into existence I never did another wirewrap proto again. Now it is even less expensive. For the same amount of work and cost I had a functioning circuit and could easily make another if needed and it looked professional.

From an out of pocket cost and EMI perspective you cannot touch Jim Williams' described cookie tin approach compared to die cast aluminum boxes. LMB and Hamilton interlocking U type boxes do not have the EMI shielding capability unless you use copper tape with conductive adhesive. Then you have the problem of opening up the box to fiddle so your back to die cast or cookie tins. As a quick test one day I placed my cell phone in an LMB U type box and screwed it closed. The phone still rang when called.

If you do not count the cost of your time then overall you cannot touch the cost and functionality of a cookie tin even if you have to buy a full one and give away the cookies.

Lastly the MayTec extrusions where the slots are cut not extruded, no matter what the width they charge $10/foot per slot so a total of ~$30/foot for that extrusion. Looks like such a deal compared to $0.80/foot. I would not want to attempt this myself on a table saw for wood. I also know people who will drive 10 miles to save $0.05/gal on gas.

I own a hacksaw (along with lots of other metal-cutting devices) and a bench vise. I'd just buy one (or 2) meter of the extruxsion with ONE slotting charge, and cut pieces to length myself. That gets the cost down to very reasonable.... You could also specify the slot width as 0.053" for 0.050 inch panel thickness. Personally, i prefer heavier gauge front panels if several controls are mounted on it, except for very small sizes of panels. Lighter gauge for sides is fine as long as nothing is affixed to them.